The electronics industry has experienced an ever increasing demand for smaller and faster electronic devices which are simultaneously able to support a greater number of increasingly complex and sophisticated functions. Accordingly, there is a continuing trend in the semiconductor industry to manufacture low-cost, high-performance, and low-power integrated circuits (ICs). Thus far these goals have been achieved in large part by scaling down semiconductor IC dimensions (e.g., minimum feature size) and thereby improving production efficiency and lowering associated costs. However, such scaling has also introduced increased complexity to the semiconductor manufacturing process. Thus, the realization of continued advances in semiconductor ICs and devices calls for similar advances in semiconductor manufacturing processes and technology.
For example, semiconductor lithography processes may use lithographic templates (e.g., photomasks or reticles) to optically transfer patterns onto a substrate. Such a process may be accomplished by projection of a radiation source, through an intervening photomask or reticle, onto the substrate having a photosensitive material (e.g., photoresist) coating. The minimum feature size that may be patterned by way of such a lithography process is limited by the wavelength of the projected radiation source. In view of this, extreme ultraviolet (EUV) light sources and lithographic processes have been introduced. In addition, EUV lithographic processes may save manufacturing cost by avoiding a need to apply a multi-patterning technique in achieving minimum feature sizes.
However, generating the EUV light (or radiation) in EUV light generation systems can be an energy intensive and difficult process to control. As merely one example, a method to produce EUV light includes utilizing a laser system to generate a laser beam to irradiate a material that in turn radiates EUV light. After amplifying the laser beam, the laser system may still have residual energy left in its gain medium. Such residual energy can be harmful to the laser system when a portion of the laser beam is reflected along the laser beam path and travels back into the gain medium. The reflected laser beam in backward direction receives residual gain from the gain medium and gets amplified. The amplified reflected laser beam may generate extra heat that requires dissipation, or may become too strong in energy level and cause damages to optical components in the laser system. Moreover, the residual gain may induce self-lasing effect in amplifier chain to affect the temporal domain performance of laser pulses in forward direction, which may further affect a target material formation when laser pulses impinge on such target material, and in turn deteriorate the EUV generation. As such, there is a great deal of interests in tools and techniques capable of accurately monitoring residual gain and/or reducing residual gain in the laser system.